With the spread of laptop computers, mobile phones, digital cameras, and the like, the demand for secondary batteries for driving these small-sized electronic devices has been expanding. Further, for these electronic devices, nonaqueous electrolyte secondary batteries (particularly lithium ion secondary batteries) have been increasingly used for their capability to achieve high capacity.
In addition to the use for small-sized electronic devices, the application of nonaqueous electrolyte secondary batteries for vehicles (EV, HV, PHV), household power supplies (HEMS), and the like, where a large amount of electric power is required, has also been considered. In this case, in an attempt to obtain a large amount of electric power, the electrode plates of a nonaqueous electrolyte secondary battery are increased in size, a large number of electrode plates are laminated to form an electrode assembly, or a large number of battery cells are assembled to form an assembled battery, for example.
In a nonaqueous electrolyte secondary battery, generally, a positive electrode plate and a negative electrode plate are laminated with a separator therebetween and housed in a case together with a nonaqueous electrolyte. The electrode plates (positive electrode plate, negative electrode plate) are each formed in a predetermined shape by applying a paste containing an electrode active material (positive electrode active material, negative electrode active material) onto the surface of a current collector made of an electrically conductive metal (metal foil), followed by drying.
In such a nonaqueous electrolyte secondary battery, when moisture is present inside the system (inside the case), with the progress of the cell reaction, gas may be generated as a side reaction.
As a technique to deal with the problem of gas generation, PTL 1 describes the formation of a coating film on the surface of the negative electrode from an additive added to the nonaqueous electrolyte.
A lithium metal oxide, which is used as an electrode active material for a nonaqueous electrolyte secondary battery, has highly Lewis basic oxygen atoms on the surface thereof. When oxygen atoms are highly Lewis basic, they are likely to react with moisture in the air (in the atmosphere). As a result of the reaction with moisture, a large number of hydroxyl groups are formed, making the binding to water (hydrogen bond) strong. Such bound water has been difficult to remove by drying for battery production (e.g., heating at 150° C. or less).
Of lithium metal oxides used as electrode active materials, metal oxides having an olivine structure (positive electrode active materials) and spinel-type lithium titanates (negative electrode active materials) have low diffusion coefficients. Therefore, it has been necessary to form such a metal oxide into nano-sized particles. This results in an increase in the specific surface area, making the problem of the reaction with moisture more prominent.
In order to ensure the electrical conductivity of an electrode active material, a technique in which a carbon composite or a carbon coating is formed on the surface of a lithium metal oxide to serve as an electrode active material has been known. In this electrode active material, it appears that the contact between the surface of the electrode active material and moisture in the atmosphere can also be prevented.
As a technique for coating the surface of an electrode active material with carbon, there is a method in which an electron-conducting substance made of an organic polymer (precursor) is mixed with particles of an electrode active material, and then the mixture is subjected to a thermal decomposition reaction to form an electron-conducting coating film.
PTL 2 describes the formation of a carbon coating film on the surface of an electrode active material (negative electrode active material: lithium titanate) using a heat treatment.
However, it has been confirmed that although the related art in which the surface of an electrode active material is coated with carbon has an improving effect on electrical conductivity, the adsorption of moisture cannot be reduced. That is, a nonaqueous electrolyte secondary battery using this electrode active material still has the problem of gas generation.
The origin of this problem is that with the related art in which the surface of an electrode active material is coated with carbon, it is difficult to uniformly coat the surface with carbon. That is, a uniform carbon coating cannot be formed, and, as a result, the electrode active material remains partially exposed, resulting in contact with moisture in the atmosphere to cause gas production.
PTL 1: JP-2013-229341 A (corresponding to US 2010/0178570 A1)
PTL 2: JP-2006-221881 A